Explaining the hobbit's small brain

A new paper published in BMC Biology may possibly reconcile the small size of the Hobbit’s brain with tool use – one of the arguments against the hobbit being a new species as it is assumed that for something as sophisticated as tool use – you’d need a pretty big brain. The study by Montgomery et al looked at the brain and body size of over 50 primate species indicate that while both brain and body size tend to grow with time, there is sufficient evidence to show that the opposite is also true, ie, shrinkage also does occur in some cases. So it looks like that it is possible for the Hobbit to have evolved to be smaller, thus strengthening the hypothesis that it’s a new species rather than a deformed human.

Background Brain size is a key adaptive trait. It is often assumed that increasing brain size was a general evolutionary trend in primates, yet recent fossil discoveries have documented brain size decreases in some lineages, raising the question of how general a trend there was for brains to increase in mass over evolutionary time. We present the first systematic phylogenetic analysis designed to answer this question. Results We performed ancestral state reconstructions of three traits (absolute brain mass, absolute body mass, relative brain mass) using 37 extant and 23 extinct primate species and three approaches to ancestral state reconstruction: parsimony, maximum likelihood and Bayesian Markov-chain Monte Carlo. Both absolute and relative brain mass generally increased over evolutionary time, but body mass did not. Nevertheless both absolute and relative brain mass decreased along several branches. Applying these results to the contentious case of Homo floresiensis, we find a number of scenarios under which the proposed evolution of the Homo floresiensis brain appears to be plausible, dependent on body mass and phylogenetic position.

Conclusions Our results confirm that brain expansion began early in primate evolution and show that increases occurred in all major clades. Only in terms of an increase in absolute mass does the human lineage appear particularly striking, with both the rate of proportional change in mass and relative brain size having episodes of greater expansion elsewhere on the primate phylogeny. However, decreases in brain mass also occurred along branches in all major clades, and we conclude that, while selection has acted to enlarge primate brains, in some lineages this trend has been reversed. Further analyses of the phylogenetic position of Homo floresiensis and better body mass estimates are required to confirm the plausibility of the evolution of its small brain mass. We find that for our dataset the Bayesian analysis for ancestral state reconstruction is least affected by inclusion of fossil data suggesting that this approach might be preferable for future studies on other taxa with a poor fossil record.